This is the hour from which you first notice you need to take a shit. If you don't shit within this hour time frame, it is possible that you will not need to shit for quite some time. So take advantage of the shit window and do not miss it, because if you do it can lead to being constipated and force you to carry this extra shit around with you for an undisclosed time. And who wants to carry around extra shit?
(similar to the caffeine window (sort of))

Golly, I missed my "shit window" and now I do not need to shit. This forces me to wait for my next shit window.